Investigation of the Dayton, Ohio, IR 75 Sign Truss Failure of September 11, 2006

On September 11, 2006, “unusual” deflections of an overhead bridge-mounted sign support truss along IR 75 in Dayton, Ohio were reported. Investigation by Ohio DOT road crews revealed the complete fracture of two truss members near one of the truss supports. While total collapse did not occur, such an event could have catastrophic consequences along a major arterial roadway like IR 75. This concern, along with the fact that a similar failure occurred near Cleveland, Ohio in 2004, prompted an investigation that is the subject of this manuscript. Results of the investigation are based upon a combination of in situ field monitoring of traffic-induced bridge vibrations at the location of the failed sign support truss, finite-element simulation of the expected dynamic response of the original truss in such an environment, the length of service of the truss at the time of its failure, the volume of truck traffic on the bridge in question during that time of service, and metallurgical examination of the failed components of the sign truss. The conclusion of the investigation is that extremely high-cycle fatigue of the chord-web diagonal welded connection was the cause of the truss failure. The in-service effective stress range of the AASHTO Category ET connection in question was most likely below the currently specified AASHTO constant amplitude fatigue limit for the detail, but the enormous quantity of response cycles (approaching or even exceeding 1 billion cycles) such a bridge-mounted sign on a heavily traveled route accumulates over a service life of 30 or 40 years, exceeds anything currently considered in the design codes for such structures. The implications of this fact on the current inventory of such structures in similar installations and of the same age range, i.e., installed with the original interstate routes, largely in the 1960’s, are obvious.